Electronic mail (e-mail) systems will, in many cases, include a feature to allow e-mail messages to be digitally signed. A digital signature is an encoded value that is included with an e-mail message to provide a recipient with information to confirm that the message is sent by a trusted sender and/or that the message is unchanged during the transmission process. Such an email system is considered a secure e-mail system. A well-known protocol for secure e-mail is the Secure Multi-Purpose Internet Mail Extension (S/MIME).
Two different types of digital signature schemes are in common use in secure e-mail: opaque and clear. An opaque signature is one where the secure e-mail has the message content contained inside the digital signature encoding. This approach, defines a signature for the message content and then encodes both that signature and the message content. The resulting encoded data is then transmitted as the e-mail message body.
A clear signature is one in which a digital signature is encoded and included with the e-mail message as an attachment. The message content is not altered and is sent in this unaltered state in the message body of the e-mail.
An advantage of an opaque e-mail message is that the content of the message is not available to recipients who are not provided with the appropriate information to decode the e-mail message body. In addition, since the message content is not immediately available, there is less of a potential for an email gateway to modify the message in some way and, in doing so, invalidate the digital signature. However, a disadvantage to the use of opaque signatures for secure e-mail is that many e-mail applications (for example Outlook 2000™ from Microsoft Corporation) are unable to display opaque signed messages in the message preview pane. To obtain the preview of the message, a user of such an e-mail application is required to open the email, thus defeating the advantages associated with the use of a preview view in the e-mail application. Further, where a receiving e-mail application does not support the protocol used to encode the e-mail, the content of an opaque signed e-mail message cannot be viewed at all.
It is therefore desirable to have a mechanism for permitting secure e-mails that are created as opaque signed to be transmitted to allow a receiving e-mail application to display such e-mails using a preview view or to allow the content of such e-mail messages to be accessible, despite a receiving e-mail application not supporting the opaque signed protocol of the sending e-mail application